Cardiovascular disease, including atherosclerosis, is the leading cause of death in the United States. The medical community has developed a number of methods and devices for treating coronary heart disease, some of which are specifically designed to treat the complications resulting from atherosclerosis and other forms of coronary arterial narrowing.
One method for treating atherosclerosis and other forms of coronary narrowing is percutaneous transluminal coronary angioplasty, commonly referred to as “angioplasty” or “PTCA”. The objective in angioplasty is to enlarge the lumen of the affected coronary artery by radial hydraulic expansion. The procedure is accomplished by inflating a balloon within the narrowed lumen of the coronary artery. Radial expansion of the coronary artery occurs in several different dimensions, and is related to the nature of the plaque. Soft, fatty plaque deposits are flattened by the balloon, while hardened deposits are cracked and split to enlarge the lumen. The wall of the artery itself is also stretched when the balloon is inflated.
Unfortunately while the affected artery can be enlarged, in some instances restenosis of the vessel occurs negating the positive effect of the angioplasty procedure. In the past, such restenosis has frequently necessitated repeat angioplasty or open heart surgery. While such restenosis does not occur in the majority of cases, it occurs frequently enough that such complications comprise a significant percentage of the overall failures of the angioplasty procedure.
To lessen the risk of restenosis, various devices have been proposed for mechanically keeping the affected vessel open after completion of the angioplasty procedure. Such endoprostheses (generally referred to as “stents”) are typically inserted into the vessel, positioned across the lesion or stenosis, and then expanded to keep the passageway clear. The stent overcomes the natural tendency of the vessel walls of some patients to restenose, thus maintaining the patency of the vessel.
Stents are delivered to the lesion, or target area, by a catheter. Typically, the stent is percutaneously introduced into the patient in an unexpanded form, having the smallest diameter possible. The small diameter is necessary during insertion in order to facilitate traversing tortuous blood vessels. When the stent reaches the target area, the stent is expanded to engage the blood vessel walls, enlarging the inner circumference of the blood vessel, and supporting the vessel wall.
The stent may be self-expanding or expanded by a number of mechanical methods, including expansion of the stent using a balloon on a balloon catheter. The balloon is inserted into the unexpanded stent, either before insertion into the patient or after the stent has reached the target site. The balloon is inflated while inside the stent, forcing the stent to expand and lodge within the blood vessel at the target site.
Stents are generally formed using any of a number of different methods. One group of stents are formed by winding a wire around a mandrel, welding or otherwise forming the stent to a desired configuration, and finally compressing the stent to an unexpanded diameter. Another group of stents are manufactured by machining tubing or solid stock material into bands, and then deforming and joining the bands to form a desired tubular configuration. Another group of stents are formed by laser etching or chemical etching, which cuts or etches a tube to a desired shape. The stent is usually etched or cut in an unexpanded state.
Helically wound stents, such as those described in U.S. Pat. No. 4,886,062 to Wiktor, the contents of which are incorporated by reference herein, generally comprise a wire formed into a waveform, such as a sinusoid, that is then helically wrapped around a mandrel to provide a tubular or cylindrical structure. Helically wound stents, however, generally include ends that are not substantially perpendicular to the longitudinal axis of the stent. In other words, due to the helical winding of the wire, a portion of each end of the stent extends further longitudinally than the remainder of each end of the stent, as shown in FIG. 2 of the Wiktor patent. In some helically wound stents, such as those described in U.S. Pat. No. 5,314,472 to Fontaine, end portions of the wire have a reduced amplitude waveform as compared to the waveforms in the middle of the wire. Wrapping such a wire around a mandrel to form a stent results in a stent with ends that may be generally perpendicular to the longitudinal axis of the stent.
Helically wound stents require a wrapping pitch or pitch angle to enable subsequent wraps of the waveform to build length of the stent without overlapping. Though the pitch angle may vary in degree of angular inclination along a length of the stent, wrap progression typically requires all pitch angles to be in the same angular direction from a reference line perpendicular to a longitudinal axis of the stent, i.e., the pitch angle(s) is/are inclined to the left of the reference line along the length of the stent or is/are inclined to the right of the reference line along the length of the stent. Having all pitch angles inclined in the same angular direction along the length of the stent may create a bias in the physical properties of the stent, which may subsequently affect the performance of the stent while being delivered and deployed in vivo.